In the manufacture of tissue products, such as facial tissue, bath tissue, paper towels and napkins, it is known to place debonders in the tissue in order to soften the feel of the tissue. Although increasing the soft feel of the tissue is advantageous, the use of debonders also tends to increase the amount of lint and slough, which is undesirable to consumers. As used herein, xe2x80x9clintxe2x80x9d is clinging bits and/or fine particles of fiber or fiber-like material not firmly bonded to the tissue that are easily released during dispensing or tissue handling. xe2x80x9cSloughxe2x80x9d is fibers, fiber debris, and fiber clumps that are left on the skin during tissue use.
Therefore, there is a need to provide a tissue sheet that not only feels soft, but also exhibits less lint and slough in use than other tissues of comparable softness.
It has now been discovered that lint and slough can be reduced, while maintaining softness, by providing a multi-ply tissue with plies having a particular layered structure. More specifically, and contrary to conventional wisdom in the art, a chemical debonder is intentionally added to the inwardly-facing layer of each of the two outer plies. Since chemically debonded layers are the primary source of lint and slough, this layering configuration traps the lint and slough in the middle of the multi-ply product, while the soft inwardly-facing layer still contributes to the overall softness. Focusing on each of the two outer plies of a two-ply or three-ply product, for example, going from the outside of the product toward the center of the product, the layered structure of the two outer plies of this invention is conceptually xe2x80x9csoft/strong/softxe2x80x9d. In comparison, the layered structure using conventional layering approach is either xe2x80x9csoft/soft/strongxe2x80x9d or xe2x80x9csoft/strong/strongxe2x80x9d. In both conventional structures, the innermost layer is a strength layer.
Hence, in one aspect the invention resides in a multi-ply tissue product comprising at least one, and particularly two, multi-layered creped outer plies of papermaking fibers, said multi-layered creped outer ply or plies having three or more layers which include a soft, outwardly-facing dryer-side layer, one or more inner strength layers and a soft, inwardly-facing air-side layer, wherein the inwardly-facing air-side layer comprises one or more chemical debonders.
In another aspect, the invention resides in a multi-ply tissue product comprising at least one, and particularly two, multi-layered creped outer plies of papermaking fibers, said multi-layered creped outer ply or plies having four or more layers which include a soft, outwardly-facing dryer-side layer, one or more inner strength layers and one or more inner air-side softness layers (hereinafter defined), wherein the inner air-side softness layer(s) comprises one or more chemical debonders. In this aspect, for a four-layer structure, it is desirable to have an untreated (no added debonder) air-side layer adjacent the inner air-side softness layer such that the basis weight of the air-side layer is less than the basis weight of the inner air-side softness layer. This puts a relatively thin untreated layer on the outside of the layered sheet that can aid in mechanical ply attachment.
These and other aspects of the invention will be further described below.
As used herein, the xe2x80x9cdryer-side layerxe2x80x9d is the outside layer of a tissue sheet or ply that contacts the dryer surface during manufacturing, such as the Yankee dryer surface, when the layered tissue sheet is dried. This layer is most effectively debonded by creping for softness and bulk generation. It is within the scope of this invention to have debonder present in the dryer-side layer, either by intentional addition or by incidental addition during formation or due to the recycle of broke. However, it is particularly advantageous to provide the softening effect solely or substantially solely by creping in order to minimize the lint/slough that can be produced by the presence of chemical debonders. In such cases, the dryer-side layer will have no debonder present or substantially no debonder present (0.005 weight percent or less).
The xe2x80x9cair-side layerxe2x80x9d is the outside layer of a tissue sheet or ply that does not contact the dryer surface during manufacturing when the layered tissue sheet is dried. For purposes of this invention, this layer is chemically debonded for softness and bulk generation.
The xe2x80x9cinner strength layerxe2x80x9d is a layer positioned between the dryer-side layer and the air-side layer of a sheet or ply. There can be one, two, three or more inner layers. Of these one or more inner layers, at least one inner layer is a strength-providing layer. This strength-providing layer is relatively strong compared to the two soft outer layers and functions to provide the tensile strength needed to process the layered sheet through the tissue machine and the converting process. It also contributes significant strength to the finished product. The added strength can be imparted by fiber selection, refining and/or the addition of chemical strength agents.
The xe2x80x9cinner air-side softness layerxe2x80x9d is an inner layer that is closer to the air side of the sheet than it is to the dryer side of the sheet, separated from the dryer side of the sheet by at least one inner strength layer, and which contains one or more chemical debonders. A four-layer example of a sheet in accordance with this embodiment of the invention would have a layer structure as follows: dryer-side layer/inner strength layer/inner air-side softness layer/any layer. The last layer (any layer) could be a softness layer, an untreated layer, or a strength layer.
As used herein, xe2x80x9cchemical strength agentsxe2x80x9d includes any chemical that increases the tensile strength of a tissue sheet. These chemicals include, without limitation, common dry strength agents and wet strength agents. By way of example, commercially available dry strength agents include Hercules Hercobond 1366, Cytec Parez 631 NC, and National Starch Redibond 5330A. Commercially available wet strength agents include Hercules Kymene 6500 and Kymene 557H.
As used herein, xe2x80x9cchemical debondersxe2x80x9d are any chemical that diminishes the capability of papermaking fibers to Hydrogen bond together, thereby reducing the strength of the resulting sheet and increasing perceived softness. Such chemical debonders include quaternary ammonium compounds, mixtures of quaternary ammonium compounds with polyhydroxy compounds, and modified polysiloxanes. Examples of quaternary ammonium compounds suitable for use in the present invention include dialkyldimethylammonium salts such as ditallow dimethyl ammonium chloride, ditallow dimethylammonium methyl sulfate, and di(hydrogenated)tallow dimethyl ammonium chloride. Particularly suitable debonders are 1-methyl-2 noroleyl-3 oleyl amidoethyl imidazolinium methyl sulfate and 1-ethyl-2 noroleyl-3 oleyl amidoethyl imidazolinium ethylsulfate. Suitable commercially chemical debonders include, without limitation, Witco Varisoft 6027 and Hercules Prosoft TQ 1003. The debonder(s) can be applied to the fibers prior to forming the sheet or after sheet formation and prior to final drying.
The amount of chemical debonder(s) added to or present in the air-side layer can be about 0.02 dry weight percent of the air-side fiber furnish or greater, more specifically from about 0.05 to about 0.5 dry weight percent, more specifically from about 0.05 to about 0.3 dry weight percent, and still more specifically from about 0.1 to about 0.2 dry weight percent.
The basis weight of each ply of the tissue product can be from about 4 pounds to about 40 pounds per 2880 square feet. The relative basis weights and fiber composition of each layer can vary widely depending upon the number of layers, the number of plies and the desired properties of the tissue product. For a two-ply facial tissue, a suitable basis weight split can be about 40 weight percent of the fibers in the dryer layer, about 25 weight percent of the fibers in the center layer, and about 35 weight percent of the fibers in the air side layer.
The total number of plies in the tissue product can be two, three, four or more. While it is convenient from a manufacturing standpoint to have all of the plies be the same, for purposes of this invention the structure and/or the composition of the various plies within the product can be the same or different. For example, the two outer plies can be layered and have three layers, while the inner ply or plies can be blended (non-layered). Alternatively, the inner ply or plies can also be layered, but have a different layer structure in terms of the total number of layers and/or the layer fiber/chemical composition than the two outer plies. A particularly suitable product configuration is a two-ply product with each ply having three layers.
Suitable fibers for the dryer-side layer include hardwood fibers, such as eucalyptus fibers, as well as softwood fibers. More specifically, the dryer-side layer can advantageously comprise from about 60 to about 100 percent hardwood fibers.
Suitable fibers for the inner strength layer also include hardwood and softwood fibers. More specifically, the inner strength layer can advantageously comprise from about 50 to about 100 percent softwood fibers.
Suitable fibers for the air-side layer include also include hardwood and softwood fibers. More specifically, the air-side layer can advantageously comprise from about 60 to about 100 percent hardwood fibers, more specifically from about 80 to about 100 percent hardwood fibers, and still more specifically from about 90 to about 100 percent hardwood fibers.
Methods for making the tissue products of this invention include any layered forming papermaking process in which the layered paper sheet is creped. Such papermaking processes include conventional wet-pressing processes and throughdrying processes. Suitable examples of such processes are described in the following U.S. patents, which are hereby incorporated by reference: U.S. Pat. No. 5,494,554, entitled xe2x80x9cMethod for Making Soft Layered Tissuesxe2x80x9d issued Feb. 27, 1996 to Edwards, et al.; U.S. Pat. No. 5,730,839, entitled xe2x80x9cMethod of Creping Tissue Webs Containing a Softener Using a Closed Creping Pocketxe2x80x9d issued Mar. 24, 1998 to Wendt, et al.; U.S. Pat. No. 3,994,771, entitled xe2x80x9cProcess for Forming a Layered Paper Web Having Improved Bulk, Tactile Impression and Absorbency and Paper Thereofxe2x80x9d issued Nov. 30, 1976 to Morgan, Jr. et al.